Fair Value Disclosures - Narrative (Details) (USD $)	6 Months Ended				3 Months Ended		6 Months Ended		0 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Level 3	Dec. 31, 2011 Level 3	Jun. 30, 2012 Gains on Trading Assets	Jun. 30, 2011 Gains on Trading Assets	Jun. 30, 2012 Gains on Trading Assets	Jun. 30, 2011 Gains on Trading Assets	Jun. 30, 2012 Asset-backed Securities Floating Interest Rate [Member]	Dec. 31, 2011 Asset-backed Securities Floating Interest Rate [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3		Jun. 30, 2011 Fair Value, Measurements, Recurring [Member] Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Reset period of auction rate certificate interest rates	28 days
Derivative asset and liability credit risk valuation adjustment	$ 700,000	$ 200,000
Assets and Liabilities, Fair Value Disclosure			6,100,000,000	5,900,000,000
Assets and liabilities fair value as percent of total assets			51.00%	49.00%
Assets and liabilities fair value as percent of total investments			70.00%	72.00%
Trading Securities, Unrealized Holding Loss									3,181,000	3,342,000	2,400,000	[1]	1,600,000	[1]	2,400,000	[1]
Changes in fair value gain (losses), fair value option					$ (3,100,000)	$ 1,300,000	$ (2,100,000)	$ 1,900,000

[1] Includes unrealized losses of $2.4 million attributable to assets still held as of June 30, 2011 that are recorded in (Losses)/gains on trading assets.